My FPS goes up by 12-20 with it underboring over my stock barrels (.690 and .691 respectively) so it does increase efficiency slightly since i can turn down my reg. I don't notice any difference with it over my freak kit. Most stock barrels are of such good quality nowadays i like it since i don't feel the need to buy yet another barrel.

Use one on my Trracer. Love it to prevent roll outs for cheaper than a frek kit and I keep the great stock barrel that fits the pump arm perfect. Never really concerned myself about efficancy as well trracers are all ready very efficient to begin with. Over 150 shots per 13/3000.